1. Field of the Invention
The present invention relates to method and apparatus for stabilizing the output frequency of an oscillator and, more particularly, to the frequency stabilization of an oscillator to compensate for slow frequency variations due to ambient temperature variations by diverting a fraction of the oscillator output to a line section having a coefficient of thermal expansion per degree Centigrade (/.degree. C.) which is greater than the coefficient of relative frequency drift/.degree. C. of the oscillator.
2. Description of the Prior Art
The stabilization of the output frequency of an oscillator has always been of importance and especially under conditions where the oscillator must operate unattended for long intervals of time during which, for example, ambient temperature fluctuations may occur as found, for instance, in communication systems.
U.S. Pat. No. 2,479,697 issued to L. E. Norton on Aug. 23, 1949 relates to a frequency stabilizer for wave generators wherein an impedance represented by a complex number is selected, transformed to a new impedance with a higher "Q" and lower absolute value, and then parallel resonated with another reactance having an opposite sign. This "synthetic" control resonator is used to stabilize the frequency of the generator. Circuit configurations are also disclosed which provide rapid rates of change of phase angle with changes in frequency.
U.S. Pat. No. Re. 23,598, reissued from U.S. Pat. No. 2,485,031 and issued to W. E. Bradley on Dec. 23, 1952, relates to a system for adjusting and stabilizing the frequency of high frequency oscillators as the voltage, current or load impedance varies. There, a parallel reactance, resonant to the frequency to be controlled, is connected across the transmission line. The reactance can be formed using a combination of network elements, coaxial lines, waveguides or resonant cavities which presents a capacitance to the line when the frequency rises in order to lower the frequency to normal and presents an inductance to the line when the frequency drops in order to raise the frequency to normal.
U.S. Pat. No. 3,617,924 issued to H. Fujita et al on Nov. 2, 1971 relates to an automatic frequency tuning system comprising an oscillator including a reference cavity, a load driven at the frequency provided from the oscillator, and a cooling line for supplying a fluid coolant from a common cooling source to both the reference cavity and the load, whereby the oscillation frequency is controlled in accordance with the temperature of the load and/or the reference cavity.
An article entitled "Transistorized Frequency Stabilization for Reflex Klystrons Used Magnetic Resonance" by P. Jung, Journal of Scientific Instruments, Vol. 37, Oct. 1960, pp. 372-374, discloses a frequency stabilizer configuration. There, the reflector voltage of the klystron is amplitude-modulated at a particular frequency. A cavity and a diode disposed at the end of the output waveguide, or in a branch thereof, acts as a frequency discriminator to transform the FM modulation into an AM wave. The output from the diode contains a component of the particular frequency which is proportional to the difference between the klystron frequency and the resonant frequency of the cavity. This error signal is amplified, demodulated and fed back to the reflector of the klystron over a lead to stabilize the oscillator frequency.
In an article "Frequency Stabilization of Klystrons," by M. J. A. Smith in Journal of Scientific Instruments, Vol. 37, Oct. 1960, pp. 398-399, an arrangement is disclosed for frequency stabilizing a klystron. There a portion of the power from the main waveguide is diverted by a directional coupler into a waveguide branch. The diverted power is mixed with a particular frequency from a separate reference source. The mixed signal next encounters a cavity tuned to one of the resulting sideband signals which signal continues to a detector crystal. The signal from the detector crystal is compared with a reference frequency and any error signal resulting from such comparison is fed to the klystron reflector over a separate lead to achieve frequency stabilization.
An article "Attainment of a Low-Noise High-Power and Highly Stable Gunn Oscillator by Coupling to a Superconducting Cavity," by J. J. Jimenez et al, Proceedings of the IEEE, Vol. 61, No. 1, January 1973, pp. 123-124 relates to a frequency stabilization circuit for a Gunn oscillator. There a high-Q superconducting cavity, which is the microwave equivalent of a low-frequency quartz filter, is coupled to a Gunn oscillator through a 10 dB coupler and a pair of phase shifters to optimize coupling. The cavity is placed in a constant pressure vessel filled with gaseous helium at a particular pressure to stabilize the cavity. The Gunn oscillator is feedback stabilized by the energy reflected from the cavity, the stabilization factor being directly proportional to the cavity Q. In the Jimenez et al arrangement, the frequency of the Gunn oscillator is determined by the frequency of the cavity which is maintained as stable as possible and unaffected by varying ambient temperatures.